SOLUTION: any AP series and find the sum of first 21 terms

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Question 998564: any AP series and find the sum of first 21 terms
Answer by ikleyn(52788) About Me  (Show Source):
You can put this solution on YOUR website!
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1, 2, 3, . . . , 21.

S = 1 + 2 + 3 + . . . + 21 = %2822%2A21%29%2F2 = 11*21 = 231.